The paper presents an improved impressed diagnostic pulse-voltage method to estimate the rotor position of a switched-reluctance motor (SRM) with an external rotor. The fundamental method of impressed diagnostic pulse voltage is to impress pulse voltage at the inactive phases of a SRM that has an external rotor. Because the variations of inductance are dependent on the rotor position, and because the characteristic current values produced from the pulse voltage are dependent on the inductance, the rotor position can be inferred by observing the characteristic-current value. If the inductance of the SRM does not vary obviously, noise in the characteristic-current signal may cause failure of the sensorless-position-estimation method. Therefore, an improved impressed diagnostic pulse-voltage method is presented to increase the variations of characteristic current and estimate rotor position. First, the structure, characteristics, operational principles and mathematical model of the SRM with an external rotor are introduced. Then the improved impressed diagnostic pulse-voltage method is utilised to implement the sensorless position estimation for commutation control